Seismic-wave attenuation and source excitation in La Paz-Los Cabos, Baja California Sur, Mexico

摘要

We present results from a regional study of seismic-wave attenuation and source excitation from small-magnitude earthquakes recorded at distances from 6 to 180 km in the La Paz-Los Cabos region, at the south end of the Baja California Peninsula. Data were recorded using 32 strong-motion seismic stations from the La Paz network (LAP). A least-squares regression separating the excitation, site, and propagation effects was carried out. We performed the analyses in the time and frequency domains, and we compared these results with results from a coda-normalization method. The propagation term was parameterized to represent a geometrical spreading function and a frequency-dependent Q(f) at a reference distance of 40 km. We estimated the regional attenuation by measuring the maximum amplitude of the S or Lg waves as a function of frequency, defining a continuous piecewise propagation term, D(r, f), after separating the excitation and site terms. Our results show that the attenuation is lower than that of central or northern Mexico. Recorded data were of remarkably good quality despite the fact that the strong-motion network recorded only small earthquakes. Our best model is that of a quality factor of Q(f) = 380f(0.10) with a complex geometrical spreading function.